Transmit diversity and spatial spreading for an OFDM-based multi-antenna communication system

ABSTRACT

A multi-antenna transmitting entity transmits data to a single- or multi-antenna receiving entity using (1) a steered mode to direct the data transmission toward the receiving entity or (2) a pseudo-random transmit steering (PRTS) mode to randomize the effective channels observed by the data transmission across the subbands. The PRTS mode may be used to achieve transmit diversity or spatial spreading. For transmit diversity, the transmitting entity uses different pseudo-random steering vectors across the subbands but the same steering vector across a packet for each subband. The receiving entity does not need to have knowledge of the pseudo-random steering vectors or perform any special processing. For spatial spreading, the transmitting entity uses different pseudo-random steering vectors across the subbands and different steering vectors across the packet for each subband. Only the transmitting and receiving entities know the steering vectors used for data transmission.

CLAIM OF PRIORITY

This application is a continuation-in-part of, and claims the benefit ofpriority from, U.S. patent application Ser. No. 10/781,951, entitled“Transmit Diversity and Spatial Spreading for an OFDM-basedMulti-Antenna Communication System” and filed Feb. 18, 2004, nowabandoned which is assigned to the assignee of this application.

BACKGROUND

1. Field

The present invention relates generally to communication, and morespecifically to techniques for transmitting data in a multi-antennacommunication system that utilizes orthogonal frequency divisionmultiplexing (OFDM).

2. Background

OFDM is a multi-carrier modulation technique that effectively partitionsthe overall system bandwidth into multiple (N_(F)) orthogonal subbands,which are also referred to as tones, subcarriers, bins, and frequencychannels. With OFDM, each subband is associated with a respectivesubcarrier that may be modulated with data. OFDM is widely used invarious wireless communication systems, such as those that implement thewell-known IEEE 802.11a and 802.1g standards. IEEE 802.11a and 802.11ggenerally cover single-input single-output (SISO) operation whereby atransmitting device employs a single antenna for data transmission and areceiving device normally employs a single antenna for data reception.

A multi-antenna communication system includes single-antenna devices andmulti-antenna devices. In this system, a multi-antenna device mayutilize its multiple antennas for data transmission to a single-antennadevice. The multi-antenna device and single-antenna device may implementany one of a number of conventional transmit diversity schemes in orderto obtain transmit diversity and improve performance for the datatransmission. One such transmit diversity scheme is described by S. M.Alamouti in a paper entitled “A Simple Transmit Diversity Technique forWireless Communications,” IEEE Journal on Selected Areas inCommunications, Vol. 16, No. 8, October 1998, pp. 1451-1458. For theAlamouti scheme, the transmitting device transmits each pair of datasymbols from two antennas in two symbol periods, and the receivingdevice combines two received symbols obtained for the two symbol periodsto recover the pair of data symbols. The Alamouti scheme as well as mostother conventional transmit diversity schemes require the receivingdevice to perform special processing, which may be different from schemeto scheme, in order to recover the transmitted data and obtain thebenefits of transmit diversity.

However, a single-antenna device may be designed for SISO operationonly, as described below. This is normally the case if the wirelessdevice is designed for the IEEE 802.11a or 802.11g standard. Such a“legacy” single-antenna device would not be able to perform the specialprocessing required by most conventional transmit diversity schemes.Nevertheless, it is still highly desirable for a multi-antenna device totransmit data to the legacy single-antenna device in a manner such thatimproved reliability and/or performance can be achieved.

There is therefore a need in the art for techniques to achieve transmitdiversity for a legacy single-antenna receiving device.

SUMMARY

Techniques for transmitting data from a multi-antenna transmittingentity to a single-antenna receiving entity using a steered mode and/ora pseudo-random transmit steering (PRTS) mode are described herein. Inthe steered mode, the transmitting entity preforms spatial processing todirect the data transmission toward the receiving entity. In the PRTSmode, the transmitting entity performs spatial processing such that thedata transmission observes random effective SISO channels across thesubbands, and performance is not dictated by a bad channel realization.The transmitting entity may use (1) the steered mode if it knows theresponse of the multiple-input single-output (MISO) channel for thereceiving entity and (2) the PRTS mode even if it does not know the MISOchannel response.

The transmitting entity performs spatial processing with (1) steeringvectors derived from the MISO channel response estimates for the steeredmode and (2) pseudo-random steering vectors for the PRTS mode. Eachsteering vector is a vector with N_(T) elements, which can be multipliedwith a data symbol to generate N_(T) transmit symbols for transmissionfrom N_(T) transmit antennas, where N_(T)>1.

The PRTS mode may be used to achieve transmit diversity withoutrequiring the receiving entity to perform any special processing. Fortransmit diversity, the transmitting entity uses (1) differentpseudo-random steering vectors across the subbands used for datatransmission and (2) the same steering vector across the pseudo-randomsteered portion of a protocol data unit (PDU) for each subband. A PDU isa unit of transmission. The receiving entity does not need to haveknowledge of the pseudo-random steering vectors used by the transmittingentity. The PRTS mode may also be used to achieve spatial spreading,e.g., for secure data transmission. For spatial spreading, thetransmitting entity uses (1) different pseudo-random steering vectorsacross the subbands and (2) different steering vectors across thepseudo-random steered portion of the PDU for each subband. For securedata transmission, only the transmitting and receiving entities know thesteering vectors used for data transmission.

The steered and PRTS modes may also be used for data transmission from amulti-antenna transmitting entity to a multi-antenna receiving entity,as described below. Various aspects and embodiments of the invention arealso described in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a multi-antenna communication system;

FIG. 2 shows a generic PDU format;

FIG. 3 shows pilot transmission from a dual-antenna transmitting entityto a single-antenna receiving entity;

FIG. 4 shows a process for transmitting data using the steered or PRTSmode;

FIG. 5 shows a process for transmitting data using both modes;

FIGS. 6A and 6B show two specific PDU formats;

FIG. 7 shows a transmitting entity and two receiving entities;

FIG. 8 shows a block diagram of a multi-antenna transmitting entity;

FIG. 9A shows a block diagram of a single-antenna receiving entity; and

FIG. 9B shows a block diagram of a multi-antenna receiving entity.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

FIG. 1 shows a multi-antenna system 100 with an access point (AP) 110and user terminals (UTs) 120. An access point is generally a fixedstation that communicates with the user terminals and may also bereferred to as a base station or some other terminology. A user terminalmay be fixed or mobile and may also be referred to as a mobile station,a wireless device, a user equipment (UE), or some other terminology. Asystem controller 130 couples to the access points and providescoordination and control for these access points.

Access point 110 is equipped with multiple antennas for datatransmission. Each user terminal 120 may be equipped with a singleantenna or multiple antennas for data transmission. A user terminal maycommunicate with the access point, in which case the roles of accesspoint and user terminal are established. A user terminal may alsocommunicate peer-to-peer with another user terminal. In the followingdescription, a transmitting entity may be an access point or a userterminal, and a receiving entity may also be an access point or a userterminal. The transmitting entity is equipped with multiple (N_(T))transmit antennas, and the receiving entity may be equipped with asingle antenna or multiple (N_(R)) antennas. A MISO transmission existswhen the receiving entity is equipped with a single antenna, and amultiple-input multiple-output (MIMO) transmission exists when thereceiving entity is equipped with multiple antennas.

System 100 may utilize a time division duplex (TDD) or a frequencydivision duplex (FDD) channel structure. For the TDD structure, thedownlink and uplink share the same frequency band, with the downlinkbeing allocated a portion of the time and the uplink being allocated theremaining portion of the time. For the FDD structure, the downlink anduplink are allocated separate frequency bands. For clarity, thefollowing description assumes that system 100 utilizes the TDDstructure.

System 100 also utilizes OFDM for data transmission. OFDM provides N_(F)total subbands, of which N_(D) subbands are used for data transmissionand are referred to as data subbands, N_(P) subbands are used for acarrier pilot and are referred to as pilot subbands, and the remainingN_(G) subbands are not used and serve as guard subbands, whereN_(F)=N_(D)+N_(P)+N_(G). In each OFDM symbol period, up to N_(D) datasymbols may be sent on the N_(D) data subbands, and up to N_(P) pilotsymbols may be sent on the N_(P) pilot subbands. As used herein, a “datasymbol” is a modulation symbol for data, and a “pilot symbol” is amodulation symbol for pilot. The pilot symbols are known a priori byboth the transmitting and receiving entities.

For OFDM modulation, N_(F) frequency-domain values (for N_(D) datasymbols, N_(P) pilot symbols, and N_(G) zeros) are transformed to thetime domain with an N_(F) point inverse fast Fourier transform (IFFT) toobtain a “transformed” symbol that contains N_(F) time-domain chips. Tocombat intersymbol interference (ISI), which is caused by frequencyselective fading, a portion of each transformed symbol is repeated toform a corresponding OFDM symbol. The repeated portion is often referredto as a cyclic prefix or guard interval. An OFDM symbol period (which isalso referred to herein as simply a “symbol period”) is the duration ofone OFDM symbol.

FIG. 2 shows an exemplary protocol data unit (PDU) format 200 that maybe used for system 100. Data is processed at a higher layer as dataunits. Each data unit 210 is coded and modulated (or symbol mapped)separately based on a coding and modulation scheme selected for thatdata unit. Each data unit 210 is associated with a signaling portion 220that carries various parameters (e.g., the rate and length) for thatdata unit, which are used by the receiving entity to process and recoverthe data unit. The signaling portion may be processed with the same ordifferent coding and modulation scheme than that used for the data unit.Each data unit and its signaling portion are OFDM modulated to form asignaling/data portion 240 of a PDU 230. The data unit is transmittedacross both subbands and symbol periods in the data portion of the PDU.PDU 230 further includes a preamble 240 that carries one or more typesof pilot used for various purposes by the receiving entity. In general,preamble 240 and signaling/data portion 250 may each be fixed orvariable length and may contain any number of OFDM symbols. PDU 230 mayalso be referred to as a packet or some other terminology.

The receiving entity typically processes each PDU separately. Thereceiving entity uses the preamble of the PDU for automatic gain control(AGC), diversity selection (to select one of several input ports toprocess), timing synchronization, coarse and fine frequency acquisition,channel estimation, and so on. The receiving entity uses the informationobtained from the preamble to process the signaling/data portion of thePDU.

In general, pseudo-random transmit steering may be applied to an entirePDU or a portion of the PDU, depending on various factors. Thepseudo-random steered portion of a PDU may thus be all or a portion ofthe PDU.

1. MISO Transmission

In system 100, a MISO channel exists between a multi-antennatransmitting entity and a single-antenna receiving entity. For anOFDM-based system, the MISO channel formed by the N_(T) antennas at thetransmitting entity and the single antenna at the receiving entity maybe characterized by a set of N_(F) channel response row vectors, each ofdimension 1×N_(T), which may be expressed as:h (k)=[h ₁(k) h ₂(k) . . . h _(N) _(T) (k)], for kεK  Eq (1)where entry h_(j)(k), for j=1 . . . N_(T), denotes the coupling orcomplex gain between transmit antenna j and the single receive antennafor subband k, and K denotes the set of N_(F) subbands. For simplicity,the MISO channel response h(k) is assumed to be constant across each PDUand is thus a function of only subband k.

The transmitting entity may transmit data from its multiple antennas tothe single-antenna receiving entity in a manner such that improvedreliability and/or performance can be achieved. Moreover, the datatransmission may be such that the single-antenna receiving entity canperform the normal processing for SISO operation (and does not need todo any other special processing for transmit diversity) to recover thedata transmission.

The transmitting entity may transmit data to the single-antennareceiving entity using the steered mode or the PRTS mode. In the steeredmode, the transmitting entity performs spatial processing to direct thedata transmission toward the receiving entity. In the PRTS mode, thetransmitting entity performs spatial processing such that the datatransmission observes random effective SISO channels across thesubbands. The PRTS mode may be used to achieve transmit diversitywithout requiring the receiving entity to perform any specialprocessing. The PRTS mode may also be used to achieve spatial spreading,e.g., for secure data transmission. Both of these modes and both ofthese applications for the PRTS mode are described below.

A. Steered Mode for MISO

The transmitting entity performs spatial processing for each subband forthe steered mode, as follows:x _(miso,sm)(n,k)= v _(sm)(k)·s(n,k),  Eq (2)where s(n,k) is a data symbol to be sent on subband k in symbol periodn;

-   -   v _(sm)(k) is an N_(T)×1 steering vector for subband k in symbol        period n; and    -   x _(miso,sm)(n,k) is an N_(T)×1 vector with N_(T) transmit        symbols to be sent from the N_(T) transmit antennas on subband k        in symbol period n.        In the following description, the subscript “sm” denotes the        steered mode, “pm” denotes the PRTS mode, “miso” denotes MISO        transmission, and “mimo” denotes MIMO transmission. With OFDM,        one substream of data symbols may be sent on each data subband.        The transmitting entity performs spatial processing for each        data subband separately.

For the steered mode, steering vectors v _(sm)(k) are derived based onthe channel response row vector h(k), as follows:v _(sm)(k)= h ^(H)(k) or v _(sm)(k)=arg{ h ^(H)(k)},  Eq (3)where arg{h ^(H)(k)} denotes the argument of h ^(H)(k) and “^(H)”denotes the complex conjugate transpose. The argument provides elementshaving unit magnitude and different phases determined by the elements ofh(k), so that the full power of each transmit antenna may be used fordata transmission. Since the channel response h(k) is assumed to beconstant across each PDU, the steering vector v _(sm)(k) is alsoconstant across the PDU and is a function of only subband k.

The received symbols at the receiving entity may be expressed as:r _(sm)(n,k)= h (k)· x _(miso,sm)(n,k)+z(n,k)= h (k)· v_(sm)(k)·s(n,k)+z(n,k)+h _(eff,sm)(k)·s(n,k)+z(n,k),  Eq(4)where r_(sm)(n,k) is a received symbol for subband k in symbol period n;

-   -   h_(eff,sm)(k) is an effective SISO channel response for subband        k, which is h_(eff,sm)(k)=h(k)·v _(sm)(k); and    -   z(n,k) is the noise for subband k in symbol period n.

As shown in equation (4), the spatial processing by the transmittingentity results in the data symbol substream for each subband k observingthe effective SISO channel response h_(eff,sm)(k), which includes theactual MISO channel response h(k) and the steering vector v _(sm)(k).The receiving entity can estimate the effective SISO channel responseh_(eff,sm)(k), for example, based on pilot symbols received from thetransmitting entity. The receiving entity can then perform detection(e.g., matched filtering) on the received symbols r_(sm)(n,k) with theeffective SISO channel response estimate, ĥ_(eff,sm)(k), to obtaindetected symbols ŝ(n,k), which are estimates of the transmitted datasymbols s(n,k).

The receiving entity may perform matched filtering as follows:

$\begin{matrix}{{{\hat{s}( {n,k} )} = {\frac{{{\hat{h}}_{{eff},{sm}}^{*}(k)} \cdot {r( {n,k} )}}{{{{\hat{h}}_{{eff},{sm}}(k)}}^{2}} = {{s( {n,k} )} + {z^{\prime}( {n,k} )}}}},} & {{Eq}\mspace{14mu}(5)}\end{matrix}$where “*” denotes a conjugate. The detection operation in equation (5)is the same as would be performed by the receiving entity for a SISOtransmission. However, the effective SISO channel response estimate,ĥ_(eff,sm)(k), is used for detection instead of a SISO channel responseestimate.

B. PRTS Mode for Transmit Diversity

For the PRTS mode, the transmitting entity uses pseudo-random steeringvectors for spatial processing. These steering vectors are derived tohave certain desirable properties, as described below.

To achieve transmit diversity with the PRTS mode, the transmittingentity uses the same steering vector across the pseudo-random steeredportion of a PDU for each subband k. The steering vectors would then bea function of only subband k and not symbol period n, or v _(pm)(k). Ingeneral, it is desirable to use as many different steering vectors aspossible across the subbands to achieve greater transmit diversity. Forexample, a different steering vector may be used for each data subband.A set of N_(D) steering vectors, denoted as {v _(pm)(k)}, may be usedfor spatial processing for the N_(D) data subbands. The same steeringvector set {v _(pm)(k)} is used for each PDU (e.g., across the preambleand signal/data portion for the PDU format shown in FIG. 2). Thesteering vector set may be the same or may change from PDU to PDU.

The transmitting entity performs spatial processing for each subband asfollows:x _(miso,pm)(n,k)= v _(pm)(k)·s(n,k).  Eq (6)One set of steering vectors {v _(pm)(k)} is used across all OFDM symbolsin the PDU.

The received symbols at the receiving entity may be expressed as:r _(td)(n,k)= h (k)· x _(miso,pm)(n,k)+z(n,k)= h (k)· v_(pm)(k)·s(n,k)+z(n,k)=h _(eff,td)(k)·s(n,k)=z(n,k).  Eq(7)

The effective SISO channel response h_(eff,td)(k) for each subband isdetermined by the actual MISO channel response h(k) for that subband andthe steering vector v _(pm)(k) used for the subband. The effective SISOchannel response h_(eff,td)(k) for each subband k is constant across thePDU because the actual channel response h(k) is assumed to be constantacross the PDU and the same steering vector v _(pm)(k) is used acrossthe PDU.

The receiving entity receives the transmitted PDU and derives aneffective SISO channel response estimate, ĥ_(eff,td)(k), for each datasubband based on the preamble. The receiving entity then uses theeffective SISO channel response estimates, ĥ_(eff,td)(k), to performdetection on the receive symbols in the signaling/data portion of thePDU, as shown in equation (5), where ĥ_(eff,td)(k) substitutes forĥ_(eff,sm)(k).

For transmit diversity, the receiving entity does not need to knowwhether a single antenna or multiple antennas are used for datatransmission, and does not need to know the steering vector used foreach subband. The receiving entity can nevertheless enjoy the benefitsof transmit diversity since different steering vectors are used acrossthe subbands and different effective SISO channels are formed for thesesubbands. Each PDU would then observe an ensemble of pseudo-random SISOchannels across the subbands used to transmit the PDU.

C. PRTS Mode for Spatial Spreading

Spatial spreading may be used to randomize a data transmission acrossspatial dimension. Spatial spreading may be used for secure datatransmission between a transmitting entity and a recipient receivingentity to prevent unauthorized reception of the data transmission byother receiving entities.

For spatial spreading in the PRTS mode, the transmitting entity usesdifferent steering vectors across the pseudo-random steered portion of aPDU for each subband k. The steering vectors would then be a function ofboth subband and symbol period, or v _(pm)(n,k). In general, it isdesirable to use as many different steering vectors as possible acrossboth subbands and symbol periods to achieve a higher degree of spatialspreading. For example, a different steering vector may be used for eachdata subband for a given symbol period, and a different steering vectormay be used for each symbol period for a given subband. A set of N_(D)steering vectors, denoted as {v(n,k)}, may be used for spatialprocessing for the N_(D) data subbands for one symbol period, and adifferent set may be used for each symbol period across the PDU. At aminimum, different sets of steering vectors are used for the preambleand the signaling/data portion of the PDU, where one set may includevectors of all ones. The steering vector sets may be the same or maychange from PDU to PDU.

The transmitting entity performs spatial processing for each subband ofeach symbol period, as follows:x _(miso,ss)(n,k)= v _(pm)(n,k)·s(n,k).  Eq (8)

The received symbols at the receiving entity may be expressed as:r _(ss)(n,k)= h (k)· x _(miso,ss)(n,k)+z(n,k)= h (k)·v_(pm)(n,k)·s(n,k)+z(n,k)=h _(eff,ss)(n,k)·s(n,k)+z(n,k).  Eq(9)The effective SISO channel response h_(eff,ss)(n,k) for each subband ofeach symbol period is determined by the actual MISO channel responseh(k) for that subband and the steering vector v(n,k) used for thesubband and symbol period. The effective SISO channel responseh_(eff,ss)(n,k) for each subband k varies across the PDU if differentsteering vectors v _(pm)(n,k) are used across the PDU.

The recipient receiving entity has knowledge of the steering vectorsused by the transmitting entity and is able to perform the complementaryspatial despreading to recover the transmitted PDU. The recipientreceiving entity may obtain this information in various manners, asdescribed below. The other receiving entities do not have knowledge ofthe steering vectors, and the PDU transmission appears spatially randomto these entities. The likelihood of correctly recovering the PDU isthus greatly diminished for these receiving entities.

The recipient receiving entity receives the transmitted PDU and uses thepreamble for channel estimation. For each subband, the recipientreceiving entity can derive an estimate of the actual MISO channelresponse (instead of the effective SISO channel response) for eachtransmit antenna, or ĥ_(j)(k) for j=1 . . . N_(T), based on thepreamble. For simplicity, channel estimation for a case with twotransmit antennas is described below.

FIG. 3 shows a model for pilot transmission on one subband k from atwo-antenna transmitting entity to a single-antenna receiving entity. Apilot symbol p(k) is spatially processed with two elements v₁(n,k) andv₂(n,k) of a steering vector v _(pm)(n,k) to obtain two transmitsymbols, which are then sent from the two transmit antennas. The twotransmit symbols observe channel responses of h₁(k) and h₂(k), which areassumed to be constant across the PDU.

If the pilot symbol p(k) is transmitted in two symbol periods using twosets of steering vectors, v _(pm)(1,k) and v _(pm)(2,k), then thereceived pilot symbols at the receiving entity may be expressed as:r(1,k)=h ₁(k)·v ₁(1,k)·p(k)+h ₂(k)·v ₂(1,k)·p(k)+z(1,k), andr(2,k)=h ₁(k)·v ₁(2,k)·p(k)+h ₂(k)·v ₂(2,k)·p(k)+z(2,k),which may be expressed in matrix form as:r _(p)(k)= V _(p)(k)· h ^(T)(k)·p(k)+ z (k)  Eq (10)where r _(p)(k)=[r_(p)(1,k) r_(p)(2,k)]^(T) is a vector with tworeceived pilot symbols for subband k, where “^(T)” denotes thetranspose;

-   -   V _(p)(k) is a matrix with the two steering vectors v        _(pm)(1,k)=[v₁(1,k) v₂(1,k)]^(T) and v _(pm)(2,k)=[v₁(2,k)        v₂(2,k)]^(T) used for subband k;    -   h(k)=[h₁(k) h₂(k)] is a channel response row vector for subband        k; and    -   z(k)=[z(1,k) z(2,k)]^(T) is a noise vector for subband k.

The receiving entity may derive an estimate of the MISO channelresponse, ĥ(k), as follows:ĥ (k)= V _(p) ⁻¹(k)· r _(p)(k)·p*(k).  Eq (11)The recipient receiving entity can compute V _(p) ⁻¹(k) since it knowsall of the elements of V _(p)(k). The other receiving entities do notknow V _(p)(k), cannot compute for V _(p) ⁻¹(k), and cannot derive asufficiently accurate estimate of h(k).

The description above is for the simple case with two transmit antennas.In general, the number of transmit antennas determines the number ofOFDM symbols for the pilot (the length of the pilot transmission) andthe size of V _(p)(k). In particular, pilot symbols are transmitted fora minimum of N_(T) symbol periods, and the matrix V _(p)(k) is typicallyof dimension N_(T)×N_(T).

The recipient receiving entity can thereafter derive an estimate of theeffective SISO channel response, ĥ_(eff,ss)(n,k), for each subsequentOFDM symbol in the PDU, as follows:ĥ _(eff,ss)(n,k)= ĥ (k)· v _(pm)(n,k).  Eq (12)The steering vector v _(pm)(n,k) may change from symbol period to symbolperiod for each subband. However, the recipient receiving entity knowsthe steering vector used for each subband and each symbol period. Thereceiving entity uses the effective SISO channel response estimate,ĥ_(eff,ss)(n,k), for each subband of each symbol period to performdetection on the received symbol for that subband and symbol period,e.g., as shown in equation (5), where ĥ_(eff,ss)(n,k) substitutes forĥ_(eff,sm)(k) and varies across the PDU.

The transmitting entity may also transmit the pilot “in the clear”without any spatial processing, but multiplying the pilot symbols foreach transmit antenna with a different orthogonal sequence (e.g., aWalsh sequence) of length N_(T) or an integer multiple of N_(T). In thiscase, the receiving entity can estimate the MISO channel response h(k)directly by multiplying the received pilot symbols with each orthogonalsequence used for pilot transmission and integrating over the length ofthe sequence, as is known in the art. Alternatively, the transmittingentity may transmit the pilot using one steering vector v _(pm)(1,k),and the receiving entity can estimate the effective MISO channelresponse as: ĥ_(eff)(1,k)=ĥ(k)·v _(pm)(1,k). The transmitting entity maythereafter transmit data using another steering vector v _(pm)(2,k), andthe receiving entity can then estimate the effective MISO channelresponse for the data as: ĥ_(eff)(2,k)=ĥ_(eff,1)(k)·v _(pm) ^(H)(1,k)·v_(pm)(2,k). The pilot transmission and channel estimation may thus beperformed in various manners for spatial spreading.

The transmitting entity can perform spatial spreading on both thepreamble and the signaling/data portion of the PDU. The transmittingentity can also perform spatial spreading on just the preamble, or justthe signaling/data portion. In any case, the spatial spreading is suchthat the channel estimate obtained based on the preamble is not accurateor valid for the signaling/data portion. Improved performance may beachieved by performing spatial spreading on at least the signaling/dataportion of the PDU so that this portion appears spatially random to theother receiving entities without knowledge of the steering vectors.

For spatial spreading, the recipient receiving entity knows thatmultiple antennas are used for data transmission and further knows thesteering vector used for each subband in each symbol period. The spatialdespreading is essentially achieved by using the proper steering vectorsto derive the effective SISO channel response estimates, which are thenused for data detection. The recipient receiving entity also enjoys thebenefits of transmit diversity since different steering vectors are usedacross the PDU. The other receiving entities do not know the steeringvectors used by the transmitting entity. Thus, their MISO channelresponse estimates are not valid for the signaling/data portion and,when used for data detection, provide degraded or corrupted detectedsymbols. Consequently, the likelihood of recovering the transmitted PDUmay be substantially impacted for these other receiving entities. Sincethe receiving entity need to perform special processing for channelestimation and detection for spatial spreading, legacy receivingentities, which are designed for SISO operation only, also cannotrecover a spatially spread data transmission.

Spatial spreading may also be performed for the steered mode and thePRTS mode by rotating the phase of each data symbol in a pseudo-randommanner that is known by both the transmitting and receiving entities.

FIG. 4 shows a flow diagram of a process 400 for transmitting data froma transmitting entity to a receiving entity using the steered or PRTSmode. Each PDU of data is processed (e.g., coded, interleaved, andsymbol mapped) to obtain a corresponding block of data symbols (block412). The block of data symbols and pilot symbols are demultiplexed ontoN_(D) data subbands to obtain N_(D) sequences of pilot and data symbolsfor the N_(D) data subbands (block 414). Spatial processing is thenperformed on the sequence of pilot and data symbols for each datasubband with at least one steering vector selected for the subband(block 416).

For the steered mode, one steering vector is used for each data subband,and the spatial processing with this steering vector steers thetransmission toward the receiving entity. For transmit diversity in thePRTS mode, one pseudo-random steering vector is used for each datasubband, and the receiving entity does not need to have knowledge of thesteering vector. For spatial spreading in the PRTS mode, at least onepseudo-random steering vector is used for each data subband, wheredifferent steering is applied to the preamble and the signaling/dataportion, and only the transmitting and receiving entities have knowledgeof the steering vector(s). For the PRTS mode, the spatial processingwith the pseudo-random steering vectors randomizes the N_(D) effectiveSISO channels observed by the N_(D) sequences of pilot and data symbolssent on the N_(D) subbands.

The receiving entity may not be able to properly process a datatransmission sent using the PRTS mode. This may be the case, forexample, if the receiving entity assumes that the channel response issomewhat correlated across the subbands and uses some form ofinterpolation across the subbands for channel estimation. In this case,the transmitting entity can transmit using a “clear” mode without anyspatial processing. The transmitting entity may also define and/orselect the steering vectors in a manner to facilitate channel estimationfor such a receiving entity. For example, the transmitting entity mayuse the same steering vector for each set of N_(x) subbands, whereN_(X)>1. As another example, the steering vectors may be defined to becorrelated (e.g., to be rotated versions of one another) across thesubbands.

D. Multi-Mode Operation

The transmitting entity may also transmit data to the receiving entityusing both the steered and PRTS modes. The transmitting entity can usethe PRTS mode when the channel response is not known and switch to thesteered mode once the channel response is known. For a TDD system, thedownlink and uplink responses may be assumed to be reciprocal of oneanother. That is, if h(k) represents the channel response row vectorfrom the transmitting entity to the receiving entity, then a reciprocalchannel implies that the channel response from the receiving entity tothe transmitting entity is given by h ^(T) (k). The transmitting entitycan estimate the channel response for one link (e.g., downlink) based ona pilot transmission sent by the receiving entity on the other link(e.g., uplink).

FIG. 5 shows a flow diagram of a process 500 for transmitting data froma transmitting entity to a receiving entity using both the steered andPRTS modes. Initially, the transmitting entity transmits data to thereceiving entity using the PRTS mode since it does not have channelresponse estimates for the receiving entity (block 512). Thetransmitting entity derives channel response estimates for the linkbetween the transmitting and receiving entities (block 514). Forexample, the transmitting entity can (1) estimate the channel responsefor a first link (e.g., the uplink) based on a pilot sent by thereceiving entity and (2) derive channel response estimates for a secondlink (e.g., the downlink) based on (e.g., as a reciprocal of) thechannel response estimates for the first link. The transmitting entitythereafter transmits data to the receiving entity using the steeredmode, with steering vectors derived from the channel response estimatesfor the second link, once the channel response estimates for thereceiving entity are available (block 516).

The transmitting entity can go back and forth between the steered andPRTS modes depending on whether or not channel response estimates areavailable. The receiving entity performs the same processing for channelestimation and detection for both modes and does not need to be aware ofwhich mode is being used by the transmitting entity for any given PDU.Better performance can typically be achieved with the steered mode, andthe transmitting entity may be able to use a higher rate for the steeredmode. In any case, the transmitting entity can signal the rate used foreach PDU in the signaling portion of the PDU. The receiving entity wouldthen process each PDU based on the channel estimates obtained for thatPDU and in accordance with the indicated rate.

2. MIMO Transmission

In system 100, a MIMO channel exists between a multi-antennatransmitting entity and a multi-antenna receiving entity. For anOFDM-based system, the MIMO channel formed by the N_(T) antennas at thetransmitting entity and the N_(R) antenna at the receiving entity may becharacterized by a set of N_(F) channel response matrices, each ofdimension N_(R)×N_(T), which may be expressed as:

$\begin{matrix}{{{\underset{\_}{H}(k)} = \begin{bmatrix}{h_{1,1}(k)} & {h_{1,2}(k)} & \ldots & {h_{1,N_{T}}(k)} \\{h_{2,1}(k)} & {h_{2,2}(k)} & \ldots & {h_{2,N_{T}}(k)} \\\vdots & \vdots & \ddots & \vdots \\{h_{N_{R},1}(k)} & {h_{N_{R},2}(k)} & \ldots & {h_{N_{R},N_{T}}(k)}\end{bmatrix}},{{{for}\mspace{14mu} k} \in K},} & {{Eq}\mspace{14mu}(13)}\end{matrix}$where entry h_(i,j)(k), for i=1 . . . N_(R) and j=1 . . . N_(T), denotesthe coupling between transmit antenna j and receive antenna i forsubband k. For simplicity, the MIMO channel response H(k) is assumed tobe constant over each PDU.

The channel response matrix H(k) for each subband may be decomposed intoN_(S) spatial channels, where N_(S)≦min {N_(T), N_(R)}. The N_(S)spatial channels may be used to transmit data in a manner to achievegreater reliability and/or higher overall throughput. For example, N_(S)data symbols may be transmitted simultaneously from the N_(T) transmitantennas in each symbol period to achieve higher throughput.Alternatively, a single data symbol may be transmitted from the N_(T)transmit antennas in each symbol period to achieve greater reliability.For simplicity, the following description assumes thatN_(S)=N_(T)≦N_(R).

The transmitting entity may transmit data to the receiving entity usingthe steered or PRTS mode. In the steered mode for MIMO, the transmittingentity performs spatial processing to transmit data symbols on the“eigenmodes” of the MIMO channel, as described below. In the PRTS mode,the transmitting entity performs spatial processing such that the datasymbols observe random effective MIMO channels. The steered and PRTSmodes use different steering matrices and require different spatialprocessing by the receiving entity. The PRTS mode may also be used fortransmit diversity and spatial spreading.

A. Steered Mode for MIMO

For the steered mode for MIMO, the transmitting entity derives steeringmatrices V _(sm)(k) by performing singular value decomposition of thechannel response matrix H(k) for each subband, as follows:H (k)= U (k)Σ(k) V _(sm) ^(H)(k),  Eq (14)where U(k) is an N_(R)×N_(R) unitary matrix of left eigenvectors ofH(k);

-   -   Σ(k) is an N_(R)×N_(T) diagonal matrix of singular values of        H(k); and    -   V _(sm)(k) is an N_(T)×N_(T) unitary matrix of right        eigenvectors of H(k).        A unitary matrix M is characterized by the property M ^(H) M=I,        where I is the identity matrix. The columns of a unitary matrix        are orthogonal to one another. Since the channel response H(k)        is assumed to be constant across a PDU, the steering matrices V        _(sm)(k) are also constant across the PDU and is a function of        only subband k.

The transmitting entity performs spatial processing for each subband asfollows:x _(mimo,sm)(n,k)= V _(sm)(k)·s(n,k),  Eq (15)where s(n,k) is an N_(T)×1 vector with N_(T) data symbols to be sent onsubband k in symbol period n; and

-   -   x _(mimo,sm)(n,k) is an N_(T)×1 vector with N_(T) transmit        symbols to be sent from the N_(T) transmit antennas on subband k        in symbol period n.        The spatial processing with the steering matrices V _(sm)(k)        results in the N_(T) data symbols in s(n,k) being transmitted on        N_(T) eigenmodes of the MIMO channel, which may be viewed as        orthogonal spatial channels.

The received symbols at the receiving entity may be expressed as:r _(sm)(n,k)= H (k)· x _(mimo,sm)(n,k)+ z (n,k)= H (k)· V _(sm)(k)· s(n,k)+ z (n,k),  Eq (16)where r _(sm)(n,k) is an N_(R)×1 vector with N_(R) received symbols forsubband k in symbol period n; and

-   -   z(n,k) is a noise vector for subband k in symbol period n.        For simplicity, the noise is assumed to be additive white        Gaussian noise (AWGN) with a zero mean vector and a covariance        matrix of Λ=σ²·I, where σ² is the variance of the noise observed        by the receiving entity.

The receiving entity performs spatial processing for the steered mode asfollows:ŝ _(sm)(n,k)=Σ ⁻¹(n,k)· U ^(H)(n,k)· r _(sm) (n,k)= s (n,k)+ z′(n,k),  Eq (17)where ŝ _(sm)(n,k) is a vector with N_(T) detected symbols for thesteered mode, which is an estimate of s(n,k), and z′(n,k) is apost-detection noise vector.

B. Steered Mode with Spatial Spreading

Spatial spreading may also be performed in combination with the steeredmode. In this case, the transmitting entity first performs spatialprocessing on the data symbol vector s(n,k) for spatial spreading andthen performs spatial processing on the resultant spread symbols for thesteered mode. For spatial spreading, the transmitting entity usesdifferent steering matrices across the pseudo-random steered portion ofa PDU for each subband k. It is desirable to use as many differentsteering matrices as possible across both subbands and symbol periods toachieve a higher degree of spatial spreading. For example, a differentset of steering matrices {V _(pm)(n,k)} may be used for each symbolperiod across the PDU. At a minimum, one steering matrix set is used forthe preamble and another steering matrix set is used for the remainderof the PDU, where one steering matrix set may include identity matrices.

The transmitting entity performs spatial processing for each subband ofeach symbol period, as follows:x _(mimo,sm,ss)(n,k)= V _(sm)(k)· V _(pm)(n,k)· s (n,k),  Eq (18)where V _(pm)(n,k) is an N_(T)×N_(T) pseudo-random steering matrix forsubband k in symbol period n. As shown in equation (18), thetransmitting entity performs spatial spreading with the pseudo-randomsteering matrix {V _(pm)(n,k)} first, followed by spatial processing forthe steered mode with the steering matrix {V _(sm)(k)} derived from theMIMO channel response matrix H(k). The spread symbols (instead of thedata symbols) are thus transmitted on the eigenmodes of the MIMOchannel.

The received symbols at the receiving entity may be expressed as:r _(sm,ss)(n,k)= H (k)· x _(mimo,sm,ss)(n,k)+ z (n,k)= H (k)· V_(sm)(k)· V _(pm)(n,k)· s (n,k)+ z (n,k).  Eq(19)

The receiving entity performs spatial processing for the steered modeand spatial despreading as follows:ŝ _(sm,ss)(n,k)= V _(pm) ^(H)(n,k)·Σ ⁻¹(n,k)· U ^(H)(n,k)· r_(sm,ss)(n,k)= s (n,k)+ z ′(n,k),  Eq (20)As shown in equation (20), the receiving entity can recover thetransmitted data symbols by first performing the receiver spatialprocessing for the steered mode followed by spatial despreading with thepseudo-random steering matrix {V _(pm)(n,k)}. For the steered mode withspatial spreading, the effective MIMO channel observed by the datasymbols for each subband includes both matrices V _(sm)(k) and V_(pm)(n,k) used by the transmitting entity.

C. PRTS Mode for Transmit Diversity

For the PRTS mode for MIMO, the transmitting entity uses pseudo-randomsteering matrices for spatial processing. These steering matrices arederived to have certain desirable properties, as described below.

To achieve transmit diversity with the PRTS mode, the transmittingentity uses different steering matrices across the subbands but the samesteering matrix across the pseudo-random steered portion of a PDU foreach subband k. It is desirable to use as many different steeringmatrices as possible across the subbands to achieve greater transmitdiversity.

The transmitting entity performs spatial processing for each subband asfollows:x _(mimo,td)(n,k)= V _(pm)(k)· s (n,k),  Eq (21)where V _(pm)(k) is an N_(T)×N_(T) steering matrix for subband k insymbol period n; and

-   -   x _(mimo,td)(n,k) is an N_(T)×1 vector with N_(T) transmit        symbols to be sent from the N_(T) transmit antennas on subband k        in symbol period n.        One set of steering matrices {V _(pm)(k)} is used across all        OFDM symbols in the PDU.

The received symbols at the receiving entity may be expressed as:r _(td)(n,k)= H (k)· x _(mimo,td)(n,k)+ z (n,k)= H (k)· V _(pm)(k)· s(n,k)+ z (n,k)= H _(eff,td)(k)· s (n,k)+ z (n,k),  Eq(22)where r _(td)(n,k) is a vector of received symbols for the PRTS mode;and

-   -   H _(eff,td)(k) is an N_(T)×N_(T) effective MIMO channel response        matrix for subband k in symbol period n, which is H        _(eff,td)(k)=H(k)·V _(pm)(k).

The spatial processing with the pseudo-random steering matrix V _(pm)(k)results in the data symbols in s(n,k) observing an effective MIMOchannel response H _(eff,td)(k), which includes the actual channelresponse H(k) and the steering matrix V _(pm)(k). The receiving entitycan estimate the effective MIMO channel response H _(eff,td)(k), forexample, based on pilot symbols received from the transmitting entity.The receiving entity can then perform spatial processing on the receivedsymbols in r _(td)(n,k) with the effective MIMO channel responseestimate, Ĥ _(eff,td)(k), to obtain detected symbols ŝ _(td) (n,k). Theeffective MIMO channel response estimate, Ĥ _(eff,td)(k), for eachsubband k is constant across the PDU because (1) the actual MIMO channelresponse H(k) is assumed to be constant across the PDU and (2) the samesteering matrix V _(pm)(k) is used across the PDU.

The receiving entity can derive the detected symbols using variousreceiver processing techniques including (1) a channel correlationmatrix inversion (CCMI) technique, which is also commonly referred to asa zero-forcing technique, and (2) a minimum mean square error (MMSE)technique. Table 1 summarizes the spatial processing at the receivingentity for the CCMI and MMSE techniques. In Table 1, M _(ccmi,td)(k) isa spatial filter matrix for the CCMI technique, M _(mmse,td)(k) is aspatial filter matrix for the MMSE technique, and D _(mmse,td)(k) is adiagonal matrix for the MMSE technique (which contains the diagonalelements of M _(mmse,td)(k)Ĥ _(eff,td)(k)).

TABLE 1 Technique Receiver Spatial Processing CCMI {circumflex over(s)}_(ccmi,td) (n,k) = M _(ccmi,td) (k) · r _(td) (n,k) SpatialProcessing M _(ccmi,td) (k) = Spatial [{circumflex over (H)}^(H)_(eff,td) (k)Ĥ _(eff,td) (k)]⁻¹{circumflex over (H)}^(H) _(eff,td) (k)Filter Matrix MMSE {circumflex over (S)}_(mmse,td) (n,k) = D ⁻¹_(mmse,td) (k) · Spatial M _(mmse,td) (k) · r _(td) (n,k) Processing M_(mmse,td) (k) = {circumflex over (H)}^(H) _(eff,td) (k) · [{circumflexover (H)}_(eff,td) (k) · Spatial {circumflex over (H)}^(H) _(eff,td)(k) + σ² · I]⁻¹ Filter D _(mmse,td) (k) = diag [M _(mmse,td)(k){circumflex over (H)}_(eff,td) (k)] Matrix

As shown in Table 1, for transmit diversity, the spatial filter matricesM _(ccmi,td)(k) and M _(mmse,td)(k) for each subband k are constantacross the PDU because the effective MIMO channel response estimate, Ĥ_(eff,td)(k), is constant across the PDU. For transmit diversity, thereceiving entity does not need to know the steering matrix used for eachsubband. The receiving entity can nevertheless enjoy the benefits oftransmit diversity since different steering matrices are used across thesubbands and different effective MIMO channels are formed for thesesubbands.

D. PRTS Mode for Spatial Spreading

For spatial spreading in the PRTS mode, the transmitting entity usesdifferent steering matrices across the pseudo-random steered portion ofa PDU for each subband k. The pseudo-random steering matrices forspatial spreading may be selected as described above for the steeredmode.

The transmitting entity performs spatial processing for each subband ofeach symbol period, as follows:x _(mimo,ss)(n,k)= V _(pm)(n,k)· s (n,k).  Eq (23)

The received symbols at the receiving entity may be expressed as:r _(ss)(n,k)= H (k)· x _(mimo,ss)(n,k)+ z (n,k)= H (k)· V _(pm)(n,k)· s(n,k)+ z (n,k)= H _(eff,ss)(n,k)· s (n,k)+ z (n,k).  Eq(24)The effective MIMO channel response H _(eff,ss)(n,k) for each subband ofeach symbol period is determined by the actual channel response H(k) forthe subband and the steering matrix V _(pm)(n,k) used for that subbandand symbol period. The effective MIMO channel response H _(eff,ss)(n,k)for each subband k varies across the PDU because different steeringmatrices V _(pm)(n,k) are used across the PDU.

The recipient receiving entity receives the transmitted PDU and uses thepreamble for channel estimation. For each subband, the recipientreceiving entity can derive an estimate of the actual MIMO channelresponse H(k) (instead of the effective MIMO channel response) based onthe preamble. The recipient receiving entity can thereafter derive anestimate of the effective MIMO channel response matrix, Ĥ_(eff,ss)(n,k), for each subband of each symbol period, as follows:Ĥ _(eff,ss)(n,k)= Ĥ (k)· V _(pm)(n,k).  Eq(25)The steering matrix V _(pm)(n,k) may change from symbol period to symbolperiod for each subband. The receiving entity uses the effective MIMOchannel response estimate, Ĥ _(eff,ss)(n,k), for each subband of eachsymbol period to perform spatial processing on the receive symbols forthat subband and symbol period, e.g., using the CCMI or MMSE technique.For example, the matrix Ĥ _(eff,ss)(n,k) may be used to derive thespatial filter matrix for the CCMI or MMSE technique, as shown in Table1, where Ĥ _(eff,ss)(n,k) substitutes for Ĥ _(eff,td)(k). However,because the matrix Ĥ _(eff,ss)(n,k) varies across the PDU, the spatialfilter matrix also varies across the PDU.

For spatial spreading, the recipient receiving entity has knowledge ofthe steering matrix used by the transmitting entity for each subband ineach symbol period and is able to perform the complementary spatialdespreading to recover the transmitted PDU. The spatial despreading isachieved by using the proper steering matrices to derive the effectiveMIMO channel response estimates, which are then used for spatialprocessing. The other receiving entities do not have knowledge of thesteering matrices and the PDU transmission appears spatially random tothese entities. As a result, these other receiving entities have a lowlikelihood of recovering the transmitted PDU.

E. Multi-Mode Operation

The transmitting entity may also transmit data to the receiving entityusing both the PRTS and steered modes. The transmitting entity can usethe PRTS mode when the channel response is not available and switch tothe steered mode once the channel response is available.

3. Steering Vector and Matrix Generation

The steering vectors and matrices used for the PRTS mode may begenerated in various manners. Some exemplary schemes for generatingthese steering vectors/matrices are described below. The steeringvectors/matrices may be pre-computed and stored at the transmitting andreceiving entities and thereafter retrieved for use as they are needed.Alternatively, these steering vectors/matrices may be computed in realtime as they are needed. In the following description, a set of Lsteering vectors or matrices is generated and selected for use for thePRTS mode.

A. Steering Vector Generation

The steering vectors used for the PRTS mode should have the followingproperties in order to achieve good performance. Strict adherence tothese properties is not necessary. First, each steering vector shouldhave unit energy so that the transmit power used for the data symbols isnot varied by the pseudo-random transmit steering. Second, the N_(T)elements of each steering vector may be defined to have equal magnitudeso that the full transmit power of each antenna can be used. Third, thedifferent steering vectors should be reasonably uncorrelated so that thecorrelation between any two steering vectors in the set is zero or a lowvalue. This condition may be expressed as:c(ij)= v _(pm) ^(H)(i)· v _(pm)(j)≈0, for i=1 . . . L, j=1 . . . L, andi≠j,  Eq (26)where c(ij) is the correlation between steering vectors v _(pm)(i) and v_(pm)(j).

The set of L steering vectors {v _(pm)(i)} may be generated usingvarious schemes. In a first scheme, the L steering vectors are generatedbased on N_(T)×N_(T) matrices G of independent identically distributed(IID) complex Gaussian random variables, each having zero mean and unitvariance. A correlation matrix of each matrix G is computed as R=G^(H)·G and decomposed as R=E·D·E ^(H) to obtain a unitary matrix E. Eachcolumn of E may be used as a steering vector v _(pm)(i) if it meets thelow correlation criterion with each of the steering vectors already inthe set.

In a second scheme, the L steering vectors are generated by successivelyrotating an initial unitary steering vector v _(pm) (1) as follows:v _(pm)(i+1)=e ^(j2π/L) ·v _(pm)(i), for i=2 . . . L, where L≧N_(T).  Eq(27)

In a third scheme, the L steering vectors are generated such that theelements of these vectors have the same magnitude but different phases.For a given steering vector v _(pm)(i)=[v₁(i) v₂(i) . . . v_(N) _(T)(i)], which may be generated in any manner, a normalized steering vector{tilde over (v)} _(pm) (i) may be formed as:v _(pm)(i)=[Ae^(jθ) ^(f) ^((i)) Ae^(jθ) ² ^((i)) . . . Ae^(jθ) ^(NT)^((i))],  Eq (28)where A is a constant (e.g., A=1/√{square root over (N_(T))}) andθ_(j)(i)=∠v_(j) (i)=tan⁻¹

$\begin{matrix}{{{{\overset{\sim}{\underset{\_}{v}}}_{pm}(i)} = \begin{bmatrix}{A\;{\mathbb{e}}^{j\;{\theta_{1}{(i)}}}} & {A\;{\mathbb{e}}^{j\;{\theta_{2}{(i)}}}} & \ldots & {A\;{\mathbb{e}}^{j\;\theta_{N_{T}{(i)}}}}\end{bmatrix}},} & {{Eq}\mspace{14mu}(28)}\end{matrix}$is the phase of the j-th element of v _(pm)(i). The normalized steeringvector {tilde over (v)} _(pm)(i) allows the full transmit poweravailable for each antenna to be used for transmission.

Other schemes may also be used to generate the set of L steeringvectors, and this is within the scope of the invention.

B. Steering Matrix Generation

The steering matrices used for the PRTS mode should have the followingproperties in order to achieve good performance. Strict adherence tothese properties is not necessary. First, the steering matrices shouldbe unitary matrices and satisfy the following condition:V _(pm) ^(H)(i)· V _(pm)(i)= I , for i=1 . . . L. Eq  (29)Equation (29) indicates that each column of V _(pm)(i) should have unitenergy and the Hermitian inner product of any two columns of V _(pm)(i)should be zero. This condition ensures that the N_(T) data symbols sentsimultaneously using the steering matrix V _(pm)(i) have the same powerand are orthogonal to one another prior to transmission. Second, thecorrelation between any two steering matrices in the set should be zeroor a low value. This condition may be expressed as:C (ij) V _(pm) ^(H)(i)· V _(pm)(j)≈0, for i=1 . . . L, j=1 . . . L, andi≠j,  Eq (30)where C(ij) is the correlation matrix for V _(pm)(i) and V _(pm)(j) and0 is a matrix of all zeros. The L steering matrices may be generatedsuch that the maximum energy of the correlation matrices for allpossible pairs of steering matrices is minimized.

The set of L steering matrices {V _(pm)(i)} may be generated usingvarious schemes. In a first scheme, the L steering matrices aregenerated based on matrices of random variables. A matrix G of randomvariables is initially generated, and a correlation matrix of G iscomputed and decomposed to obtain a unitary matrix E, as describedabove. If low correlation exists between E and each of the steeringmatrices already generated, then E may be used as a steering matrix V_(pm)(i) and added to the set. The process is repeated until all Lsteering matrices are generated.

In a second scheme, the L steering matrices are generated bysuccessively rotating an initial unitary matrix V(1) in anN_(T)-dimensional complex space, as follows:V _(pm)(i+1)=Θ ^(i) V _(pm)(1), for i=1 . . . L−1,  Eq (31)where Θ is an N_(T)×N_(T) diagonal unitary matrix with elements that areL-th roots of unity. The second scheme is described by B. M. Hochwald etal. in “Systematic Design of Unitary Space-Time Constellations,” IEEETransaction on Information Theory, Vol. 46, No. 6, September 2000.

Other schemes may also be used to generate the set of L steeringmatrices, and this is within the scope of the invention. In general, thesteering matrices may be generated in a pseudo-random or deterministicmanner.

C. Steering Vector/Matrix Selection

The L steering vectors/matrices in the set may be selected for use invarious manners. A steering vector may be viewed as a degeneratedsteering matrix containing just one column. Thus, as used herein, amatrix may contain one or multiple columns.

In one embodiment, the steering matrices are selected from the set of Lsteering matrices in a deterministic manner. For example, the L steeringmatrices may be cycled through and selected in sequential order,starting with V(1), then V(2), and so on, and then V(L). In anotherembodiment, the steering matrices are selected from the set in apseudo-random manner. For example, the steering matrix to use for eachsubband k may be selected based on a function ƒ(k) that pseudo-randomlyselects one of the L steering matrices, or V(ƒ(k)). In yet anotherembodiment, the steering matrices are selected from the set in a“permutated” manner. For example, the L steering matrices may be cycledthrough and selected for use in sequential order. However, the startingsteering matrix for each cycle may be selected in a pseudo-randommanner, instead of always being the first steering matrix V(1). The Lsteering matrices may also be selected in other manners.

The steering matrix selection may also be dependent on the number ofsteering matrices (L) in the set and the number of subbands (N_(M)) toapply pseudo-random transmit steering, e.g., N_(M)=N_(D)+N_(P). Ingeneral, L may be greater than, equal to, or less than N_(M). IfL=N_(M), then a different steering matrix may be selected for each ofthe N_(M) subbands. If L<N_(M), then the steering matrices are reusedfor each symbol period. If L>N_(M), then a subset of the steeringmatrices is used for each symbol period. For all cases, the N_(M)steering matrices for the N_(M) subbands may be selected in adeterministic, pseudo-random, or permutated manner, as described above.

For transmit diversity, N_(M) steering matrices are selected for theN_(M) subbands for each PDU. For spatial spreading, N_(M) steeringmatrices may be selected for the N_(M) subbands for each symbol periodof the PDU. A different set of N_(M) steering matrices may be selectedfor each symbol period, where the set may include a differentpermutation of the L steering matrices.

For spatial spreading for both MISO and MIMO, only the transmitting andreceiving entities know the pseudo-random steering matrices used forspatial processing. This may be achieved in various manners. In oneembodiment, steering matrices are pseudo-randomly selected from the setof L steering matrices based on an algorithm may be seeded with secureinformation (e.g., a key, a seed, an identifier, or a serial number)exchanged between the transmitting and receiving entities (e.g., viasecure over-the-air signaling or by some other means). This results inthe set of steering matrices being permutated in a manner known only tothe transmitting and receiving entities. In another embodiment, thetransmitting and receiving entities modify the common steering matricesknown to all entities using a unique matrix U _(u) that is known only tothe two entities. This operation may be expressed as: V _(pm,u)(i)=U_(u)·V _(pm) (i) or v _(pm,u)(i)=U _(u)·v _(pm)(i). The modifiedsteering matrices are then used for spatial processing. In yet anotherembodiment, the transmitting and receiving entities permutate thecolumns of the common steering matrices in a manner known only to thesetwo entities. In yet another embodiment, the transmitting and receivingentities generate the steering matrices as they are needed based on somesecure information known only to these two entities. The pseudo-randomsteering matrices used for spatial spreading may be generated and/orselected in various other manners, and this is within the scope of theinvention.

4. IEEE 802.11

The techniques described herein may be used for various OFDM systems,e.g., for systems that implement IEEE 802.11a and 802.11g. The OFDMstructure for 802.11a/g partitions the overall system bandwidth into 64orthogonal subbands (or N_(F)=64), which are assigned indices of −32 to+31. Of these 64 subbands, 48 subbands (with indices of ±{1, . . . , 6,8, . . . , 20, 22, . . . , 26}) are used for data transmission, foursubbands (with indices of ±{7, 21}) are used for pilot transmission, andthe DC subband (with index of 0) and the remaining subbands are not usedand serve as guard subbands. For IEEE 802.11a/g, each OFDM symbol iscomposed of a 64-chip transformed symbol and a 16-chip cyclic prefix.IEEE 802.11a/g uses a 20 MHz system bandwidth. Thus, each chip has aduration of 50 nsec, and each OFDM symbol has a duration of 4.0 μsec,which is one OFDM symbol period for this system. This OFDM structure isdescribed in a document for IEEE Standard 802.11a entitled “Part 11:Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)Specifications: High-speed Physical Layer in the 5 GHz Band,” September1999, which is publicly available.

FIG. 6A shows a PDU format 600 defined by IEEE 802.11. Format 600supports both the steered mode and the PRTS mode (for both transmitdiversity and spatial spreading) for MISO transmission. At a physical(PHY) layer in the protocol stack for IEEE 802.11, data is processed asPHY sublayer service data units (PSDUs). Each PSDU 630 is coded andmodulated separately based on a coding and modulation scheme selectedfor that PSDU. Each PSDU 630 further has a PLCP header 610 that includessix fields. A rate field 612 indicates the rate for the PSDU. A reservedfield 614 includes one reserved bit. A length field 616 indicates thelength of the PSDU in units of octets. A parity field 618 carries a1-bit even parity for the three preceding fields. A tail field 620carries six zeros used to flush out the encoder. A service field 622includes seven null bits used to initialize a scrambler for the PSDU andnine reserved bits. A tail field 632 is appended at the end of PSDU 630and carries six zeros used to flush out the encoder. A variable lengthpad field 634 carries a sufficient number of pad bits to make the PSDUfit an integer number of OFDM symbols.

Each PSDU 630 and its associated fields are transmitted in one PHYprotocol data unit (PPDU) 640 that includes three sections. A preamblesection 642 has a duration of four OFDM symbol periods and carries tenshort training symbols 642 a and two long training symbols 642 b, whichare used for AGC, timing acquisition, coarse and fine frequencyacquisition, channel estimation, and other purposes by a receivingentity. The ten short training symbols are generated with 12 specificpilot symbols on 12 designated subbands and span two OFDM symbolperiods. The two long training symbols are generated with 52 specificpilot symbols on 52 designated subbands and also span two OFDM symbolperiods. A signal section 644 carries one OFDM symbol for the first fivefields of the header. A data section 648 carries a variable number ofOFDM symbols for the service field of the header, the PSDU, and thesubsequent tail and pad fields. PPDU 640 may also be referred to as apacket or some other terminology.

FIG. 6B shows an exemplary PDU format 602 that supports both the steeredand PRTS modes for both MISO and MIMO transmissions. A PPDU 650 for thisformat includes a preamble section 652, a signal section 654, a MIMOpilot section 656, and a data section 658. Preamble section 652 carriesten short training symbols 652 a and two long training symbols 652 b,similar to preamble section 642. Signal section 654 carries signalingfor PPDU 650 and may be defined as shown in Table 2.

TABLE 2 Length Field (bits) Description CCH Rate Indicator 2 Rate forcontrol channel (CCH). MIMO Pilot Length 1 Length of MIMO pilot section(e.g., 2 or 4 OFDM symbol periods). MIMO Indicator 1 Indicates PLCPheader of format 602. QoS 2 Quality of service (video/voice) LengthIndicator 10 Length of data section (e.g., in multiples of the cyclicprefix length, or 800 nsec for IEEE 802.11). Rate Vector 16 Rates usedfor spatial channels 1, 2, 3, 4. Reserved 2 Reserved for future use. CRC8 CRC value for the PLCP header. Tail 6 Six zeros to flush out theencoder.Table 2 shows an exemplary format for signal section 654 for fourtransmit antennas (N_(T)=4). Up to four spatial channels may beavailable for data transmission depending on the number of receiveantennas. The rate for each spatial channel is indicated by the ratevector field. The receiving entity may determine and send back themaximum rates supported by the spatial channels. The transmitting entitymay then select the rates for data transmission based on (e.g., lessthan or equal to) these maximum rates. Other formats with differentfields may also be used for signal section 654.

MIMO pilot section 656 carries a MIMO pilot used by the receiving entityto estimate the MIMO channel. The MIMO pilot is a pilot transmitted fromall N_(T) transmit antennas (1) “in the clear” without any spatialprocessing, (2) with pseudo-random steering as shown in equation (21) or(23), or (3) on the eigenmodes of the MIMO channel as shown in equation(18). The transmit symbols for each transmit antenna for the MIMO pilotare further multiplied (or covered) with an N_(T)-chip orthogonalsequence (e.g., a 4-chip Walsh code) assigned to that transmit antenna.Data section 658 carries a variable number of OFDM symbols for the data,pad bits, and tail bits, similar to data section 648.

Pseudo-random transmit steering may be performed in various manners forformats 600 and 602. In an embodiment for the PRTS mode, pseudo-randomtransmit steering is applied across an entire PDU. In another embodimentfor the PRTS mode, pseudo-random transmit steering is applied across aportion of a PDU. For example, pseudo-random transmit steering may beapplied across the entire PDU except for the ten short training symbolsfor formats 600 and 602. Pseudo-random transmit steering on the tenshort training symbols may adversely impact signal detection, AGC,timing acquisition, and coarse frequency acquisition, and is thus notapplied on these symbols if such is the case. For transmit diversity,for each subband, the same pseudo-random steering vector/matrix is usedacross the pseudo-random steered portion of the PDU. For spatialspreading, for each subband, different vectors/matrices may be usedacross the pseudo-random steered portion of the PDU. At a minimum,different steering vectors/matrices are used for the preamble/pilotportion used for channel estimation (e.g., the two long trainingsymbols) and the data section of the PDU. For format 600, differentsteering vectors may be used for the two long training symbols in thepreamble section and the data section of PPDU 640, where the steeringvector for one section may be all ones. For format 602, differentsteering matrices may be used for the MIMO pilot section and the datasection of PPDU 650, where the steering matrix for one section may bethe identity matrix.

The receiving entity typically processes each PPDU separately. Thereceiving entity can use (1) the short training symbols for AGC,diversity selection, timing acquisition, and coarse frequencyacquisition, and (2) the long training symbols for fine frequencyacquisition. The receiving entity can use the long training symbols forMISO channel estimation and the MIMO pilot for MIMO channel estimation.The receiving entity can derive the effective channel response estimatesdirectly or indirectly from the preamble or MIMO pilot and use thechannel estimates for detection or spatial processing, as describedabove.

5. System

FIG. 7 shows a block diagram of a multi-antenna transmitting entity 710,a single-antenna receiving entity 750 x, and a multi-antenna receivingentity 750 y in system 100. Transmitting entity 710 may be an accesspoint or a multi-antenna user terminal. Each receiving entity 750 mayalso be an access point or a user terminal.

At transmitting entity 710, a transmit (TX) data processor 720 processes(e.g., codes, interleaves, and symbol maps) each packet of data toobtain a corresponding block of data symbols. A TX spatial processor 730receives and demultiplexes pilot and data symbols onto the propersubbands, performs spatial processing for the steered and/or PRTS mode,and provides N_(T) streams of transmit symbols to N_(T) transmitterunits (TMTR) 732 a through 732 t. Each transmitter unit 732 processesits transmit symbol stream to generate a modulated signal. Transmitterunits 732 a through 732 t provide N_(T) modulated signals fortransmission from N_(T) antennas 734 a through 734 t, respectively.

At single-antenna receiving entity 750 x, an antenna 752 x receives theN_(T) transmitted signals and provides a received signal to a receiverunit (RCVR) 754 x. Receiver unit 754 x performs processing complementaryto that performed by transmitter units 732 and provides (1) receiveddata symbols to a detector 760× and (2) received pilot symbols to achannel estimator 784 x within a controller 780 x. Channel estimator 784x derives channel response estimates for the effective SISO channelsbetween transmitting entity 710 and receiving entity 750 x for all datasubbands. Detector 760 x performs detection on the received data symbolsfor each subband based on the effective SISO channel response estimatefor that subband and provides a stream of detected symbols for allsubbands. A receive (RX) data processor 770 x then processes (e.g.,symbol demaps, deinterleaves, and decodes) the detected symbol streamand provides decoded data for each data packet.

At multi-antenna receiving entity 750 y, N_(R) antennas 752 a through752 r receive the N_(T) transmitted signals, and each antenna 752provides a received signal to a respective receiver unit 754. Eachreceiver unit 754 processes a respective received signal and provides(1) received data symbols to a receive (RX) spatial processor 760 y and(2) received pilot symbols to a channel estimator 784 y within acontroller 780 y. Channel estimator 784 y derives channel responseestimates for the actual or effective MIMO channels between transmittingentity 710 and receiving entity 750 y for all data subbands. Controller780 y derives spatial filter matrices based on the MIMO channel responseestimates and the steering matrices and in accordance with, e.g., theCCMI or MMSE technique. RX spatial processor 760 y performs spatialprocessing on the received data symbols for each subband with thespatial filter matrix derived for that subband and provides detectedsymbols for the subband. An RX data processor 770 y then processes thedetected symbols for all subbands and provides decoded data for eachdata packet.

Controllers 740, 780 x, and 780 y control the operation of theprocessing units at transmitting entity 710 and receiving entities 750 xand 750 y, respectively. Memory units 742, 782 x, and 782 y store dataand/or program code used by controllers 740, 780 x, and 780 y,respectively. For example, these memory units may store the set of Lpseudo-random steering vectors (SV) and/or steering matrices (SM).

FIG. 8 shows an embodiment of the processing units at transmittingentity 710. Within TX data processor 720, an encoder 822 receives andencodes each data packet separately based on a coding scheme andprovides code bits. The coding increases the reliability of the datatransmission. The coding scheme may include cyclic redundancy check(CRC), convolutional, Turbo, low-density parity check (LDPC), block, andother coding, or a combination thereof. In the PRTS mode, the SNR canvary across a data packet even if the wireless channel is flat acrossall subbands and static over the data packet. A sufficiently powerfulcoding scheme may be used to combat the SNR variation across the datapacket, so that coded performance is proportional to the average SNRacross the data packet. An interleaver 824 interleaves or reorders thecode bits for each data packet based on an interleaving scheme toachieve frequency, time and/or spatial diversity. A symbol mapping unit826 maps the interleaved bits for each data packet based on a modulationscheme (e.g., QPSK, M-PSK, or M-QAM) and provides a block of datasymbols for the data packet. The coding and modulation schemes used foreach data packet are determined by the rate selected for that packet.

Within TX spatial processor 730, a demultiplexer (Demux) 832 receivesand demultiplexes the block of data symbols for each data packet intoN_(D) data symbol sequences for the N_(D) data subbands. For each datasubband, a multiplexer (Mux) 834 receives pilot and data symbols for thesubband, provides the pilot symbols during the preamble and MIMO pilotportions, and provides the data symbols during the signaling and dataportions. For each data packet, N_(D) multiplexers 834 a through 834 ndprovide N_(D) sequences of pilot and data symbols for the N_(D) datasubbands to N_(D) TX subband spatial processors 840 a through 840 nd.Each spatial processor 840 performs spatial processing for the steeredor PRTS mode for a respective data subband. For MISO transmission, eachspatial processor 840 performs spatial processing on its pilot and datasymbol sequence with one or more steering vectors selected for thesubband and provides N_(T) sequences of transmit symbols for the N_(T)transmit antennas to N_(T) multiplexers 842 a through 842 t. For MIMOtransmission, each spatial processor 840 demultiplexes its pilot anddata symbol sequence into N_(S) sub-sequences for N_(S) spatialchannels, performs spatial processing on the N_(S) pilot and data symbolsub-sequences with one or more steering matrices selected for thesubband, and provides N_(T) transmit symbol sequences to N_(T)multiplexers 842 a through 842 t. Each multiplexer 842 provides asequence of transmit symbols for all subbands to a respectivetransmitter unit 732. Each transmitter unit 732 includes (1) an OFDMmodulator (MOD) 852 that performs OFDM modulation on a respective streamof transmit symbols and (2) a TX RF unit 854 that conditions (e.g.,converts to analog, filters, amplifies, and frequency upconverts) thestream of OFDM symbols from OFDM modulator 852 to generate a modulatedsignal.

FIG. 9A shows an embodiment of the processing units at single-antennareceiving entity 750 x. Receiver unit 754 x includes (1) an RX RF unit912 that conditions and digitizes the received signal from antenna 752 xand provides samples and (2) an OFDM demodulator (DEMOD) 914 thatperforms OFDM demodulation on the samples, provides received datasymbols to detector 760 x, and provides received pilot symbols tochannel estimator 784 x. Channel estimator 784 x derives the channelresponse estimates for the effective SISO channels based on the receivedpilot symbols and possibly the steering vectors.

Within detector 760 x, a demultiplexer 922 demultiplexes the receiveddata symbols for each data packet into N_(D) received data symbolsequences for the N_(D) data subbands and provides the N_(D) sequencesto N_(D) subband detectors 924 a through 924 nd. Each subband detector924 performs detection on the received data symbols for its subband withthe effective SISO channel response estimate for that subband andprovides detected symbols. A multiplexer 926 multiplexes the detectedsymbols for all data subbands and provides a block of detected symbolsfor each data packet to RX data processor 770 x. Within RX dataprocessor 770 x, a symbol demapping unit 932 demaps the detected symbolsfor each data packet in accordance with the modulation scheme used forthat packet. A deinterleaver 934 deinterleaves the demodulated data in amanner complementary to the interleaving performed on the data packet. Adecoder 936 decodes the deinterleaved data in a manner complementary tothe encoding performed on the data packet. For example, a Turbo decoderor a Viterbi decoder may be used for decoder 936 if Turbo orconvolutional coding, respectively, is performed by transmitting entity710.

FIG. 9B shows an embodiment of the processing units at multi-antennareceiving entity 750 y. Receiver units 754 a through 754 r condition,digitize, and OFDM demodulate the N_(R) received signals, providereceived data symbols to RX spatial processor 760 y, and providereceived pilot symbols to channel estimator 784 y. Channel estimator 784y derives channel response estimates for the MIMO channels based on thereceived pilot symbols. Controller 780 y derives spatial filter matricesbased on the MIMO channel response estimates and the steering matrices.Within RX spatial processor 760 y, N_(R) demultiplexers 942 a through942 r obtain the received data symbols from N_(R) receiver units 754 athrough 754 r. Each demultiplexer 942 demultiplexes the received datasymbols for each data packet into N_(D) received data symbol sequencesfor the N_(D) data subbands and provides the N_(D) sequences to N_(D) RXsubband spatial processors 944 a through 944 nd. Each spatial processor944 performs receiver spatial processing on the received data symbolsfor its subband with the spatial filter matrix for that subband andprovides detected symbols. A multiplexer 946 multiplexes the detectedsymbols for all subbands and provides a block of detected symbols foreach data packet to RX data processor 770 y, which may be implementedwith the same design as RX data processor 770 x in FIG. 9A.

The data transmission techniques described herein may be implemented byvarious means. For example, these techniques may be implemented inhardware, software, or a combination thereof. For a hardwareimplementation, the processing units used to perform or support the datatransmission techniques at the transmitting and receiving entities maybe implemented within one or more application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), fieldprogrammable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, other electronic units designed toperform the functions described herein, or a combination thereof.

For a software implementation, the data transmission techniques may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The software code may be storedin a memory unit (e.g., memory units 742, 782× and 782 y in FIG. 7) andexecuted by a processor (e.g., controllers 740, 780× and 780 y in FIG.7). The memory unit may be implemented within the processor or externalto the processor, in which case it can be communicatively coupled to theprocessor via various means as is known in the art.

Headings are included herein for reference and to aid in locatingcertain sections. These headings are not intended to limit the scope ofthe concepts described therein under, and these concepts may haveapplicability in other sections throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method of transmitting data with a transmitting entity in awireless multi-antenna communication system utilizing orthogonalfrequency division multiplexing (OFDM), the method comprising:processing a data packet to obtain a block of data symbols at thetransmitting entity; demultiplexing pilot symbols and the block of datasymbols using OFDM onto a plurality of subbands to obtain, for the datapacket, a plurality of sequences of pilot and data symbols for theplurality of subbands; and performing spatial processing, at thetransmitting entity, on at least one of the pilot and data symbols foreach subband with at least one steering vector selected for the subband,the spatial processing randomizing a plurality of effective single-inputsingle-output (SISO) channels observed across the plurality of subbands.2. The method of claim 1, wherein the pilot and data symbols for eachsubband is spatially processed with one steering vector selected for thesubband.
 3. The method of claim 2, wherein a plurality of differentsteering vectors is used for the plurality of subbands.
 4. The method ofclaim 2, wherein the one steering vector used for the spatial processingof each subband is unknown to the receiving entity.
 5. The method ofclaim 1, wherein the pilot and data symbols for each subband isspatially processed with at least two steering vectors selected for thesubband.
 6. The method of claim 1, wherein one pilot or data symbol issent on each subband in each symbol period, and wherein the pilot anddata symbols for each subband is spatially processed with a differentsteering vector for each symbol period.
 7. The method of claim 6,further comprising: selecting a steering vector for each subband in eachsymbol period from among a set of L steering vectors, where L is aninteger greater than one.
 8. The method of claim 1, wherein the at leastone steering vector used for spatial processing for each subband isknown only to the transmitting entity and the receiving entity.
 9. Themethod of claim 1, wherein the spatial processing with the at least onesteering vector for each subband is performed only on data symbols. 10.The method of claim 1, wherein the pilot symbols are used for channelestimation by the receiving entity.
 11. The method of claim 1, whereinthe data packet processing includes encoding the data packet inaccordance with a coding scheme to obtain coded data, interleaving thecoded data to obtain interleaved data, and symbol mapping theinterleaved data in accordance with a modulation scheme to obtain theblock of data symbols.
 12. The method of claim 1, further comprising:selecting the at least one steering vector for each subband from among aset of L steering vectors, where L is an integer greater than one. 13.The method of claim 12, wherein the L steering vectors are such that anypair of steering vectors among the L steering vectors have lowcorrelation.
 14. The method of claim 1, wherein each steering vectorincludes T elements having same magnitude but different phases, where Tis the number of transmit antennas at the transmitting entity and is aninteger greater than one.
 15. The method of claim 1, wherein the datapacket is a protocol data unit (PDU).
 16. The method of claim 1, whereinrandomizing the plurality of effective SISO channels enables thetransmitter to transmit secure data transmissions to a receiver.
 17. Anapparatus in a wireless multi-antenna communication system utilizingorthogonal frequency division multiplexing (OFDM), comprising: a dataprocessor operative to process a data packet to obtain a block of datasymbols; a demultiplexer operative to demultiplex pilot symbols and theblock of data symbols using OFDM onto a plurality of subbands to obtain,for the data packet a plurality of sequences of pilot and data symbolsfor the plurality of subbands; and a spatial processor operative toperform spatial processing on at least one of the pilot and data symbolsfor each subband with at least one steering vector selected for thesubband, the spatial processing randomizing a plurality of effectivesingle-input single-output (SISO) channels observed across the pluralityof subbands.
 18. The apparatus of claim 17, wherein the spatialprocessor is operative to spatially process the pilot and data symbolsfor each subband with one steering vector selected for the subband. 19.The apparatus of claim 17, wherein the spatial processor is operative tospatially process the pilot and data symbols for each subband with atleast two steering vectors selected for the subband.
 20. The apparatusof claim 19, wherein the at least two steering vectors for each subbandare known only to a transmitting entity and a receiving entity for thedata packet.
 21. The apparatus of claim 17, wherein each steering vectorincludes T elements having same magnitude but different phases, where Tis the number of antennas used to transmit the data packet and is aninteger greater than one.
 22. The apparatus of claim 17, wherein thedata packet is a protocol data unit (PDU).
 23. The apparatus of claim17, wherein randomizing the plurality of effective SISO channels enablesthe transmitter to transmit secure data transmissions to a receiver. 24.An apparatus in a wireless multi-antenna communication system utilizingorthogonal frequency division multiplexing (OFDM), comprising: means forprocessing a data packet to obtain a block of data symbols; ademultiplexer means for demultiplexing pilot symbols and the block ofdata symbols using OFDM onto a plurality of subbands to obtain, for thedata packet, a plurality of sequences of pilot and data symbols for theplurality of subbands; and a processor means for performing spatialprocessing on at least one of the pilot and data symbols for eachsubband with at least one steering vector selected for the subband, thespatial processing randomizing a plurality of effective single-inputsingle-output (SISO) channels observed across the plurality of subbands.25. The apparatus of claim 17 or 24, further comprising an antennacoupled to the processor for receiving and/or transmitting data.
 26. Theapparatus of claim 24, wherein the pilot and data symbols for eachsubband is spatially processed with one steering vector selected for thesubband.
 27. The apparatus of claim 24, wherein the pilot and datasymbols for each subband is spatially processed with at least twosteering vectors selected for the subband.
 28. The apparatus of claim27, wherein the at least two steering vectors for each subband are knownonly to a transmitting entity and a receiving entity for the datapacket.
 29. The apparatus of claim 24, wherein each steering vectorincludes T elements having same magnitude but different phases, where Tis the number of antennas used to transmit the data packet and is aninteger greater than one.
 30. The apparatus of claim 24, wherein thedata packet is a protocol data unit (PDU).
 31. The apparatus of claim24, wherein randomizing the plurality of effective SISO channels enablesthe transmitter to transmit secure data transmissions to a receiver. 32.A memory unit for processing data for transmission from transmittingdata from a transmitting entity to a receiving entity in a wirelessmulti-antenna communication system utilizing orthogonal frequencydivision multiplexing (OFDM) comprising a memory, the memory havinginstructions stored thereon, the instructions being executable by one ormore processors and the instructions comprising: instructions forprocessing a data packet to obtain a block of data symbols; instructionsfor demultiplexing pilot symbols and the block of data symbols usingOFDM onto a plurality of subbands to obtain, for the data packet, aplurality of sequences of pilot and data symbols for the plurality ofsubbands; and instructions for performing spatial processing on at leastone of the pilot and data symbols for each subband with at least onesteering vector selected for the subband, the spatial processingrandomizing a plurality of effective single-input single-output (SISO)channels observed across the plurality of subbands.
 33. The memory unitof claim 32, wherein the data packet is a protocol data unit (PDU). 34.The memory unit of claim 32, wherein randomizing the plurality ofeffective SISO channels enables the transmitter to transmit secure datatransmissions to a receiver.